Crops. 177 



sulphur trioxide and magnesia are quite evenly 

 divided between the two parts of the crop. 



Nitrogen and phosphoric acid are probably the plant food con- 

 stituents most frequently lacking in soils and in many cases their 

 depletion is to be attributed to continuous raising and selling of 

 grain crops. It is evident that either the manure from grains 

 fed on the farm should be carefully husbanded, or equivalent re- 

 turns of plant food to the farm should be made by the purchase 

 of feeding stuffs or fertilizers. This subject has been fully dis- 

 cussed in the chapter on Manures. It applies with particular 

 emphasis to cereal crops, because they are wholly dependent up- 

 on stores of available nitrogen in the soil for their supply of this 

 element arid generally thrive best when supplied with available 

 forms of phosphoric acid. 



The conservation of the smaller amounts of plant food in cereal 

 straws likewise should not be neglected. The practice of dis- 

 posing of these straws by burning is a wasteful one, for by this 



?atment the nitrogen which they contain is entirely lost. 



Food requirements of the common grasses. The common 



lys, represented in our table by meadow hay, are essentially 

 straw crops, and their food requirements practically duplicate 

 those of the cereal crops. Hays of the legumes show marked 

 differences from the true hays. While for example, clover hay 

 removes twice as much nitrogen from the land as do the cereal 

 crops or meadow hay, it should be borne in mind that, like other 

 legumes, this crop obtains almost all its nitrogen from the air 

 through the activity of bacteria living in association with its 

 roots. As will be demonstrated further on, these crops increase 

 rather than diminish the supply of nitrogen in the soil. 



The true legume hays develop extensive root systems and draw 

 heavily upon the ash constituents of the soil. This applies in a 

 limited degree to phosphoric acid, but more particularly to potash 

 and lime, which form one-half the total ash of the bean crop, two- 

 thirds of the ash of clover hay, and nearly as large a proportion 

 in the case of alfalfa hay. The legume family of plants is es- 



